Carrier amplitude-control system



1 May 1927 w. H. T. HOLDEN CARRIER AMPLITUDE CONTROL SYSTEM Filed March 1926 NNN A TTORNE Y Patented May 17, 1927.

UNITED STATES PATENT OFFICE.

WILLIAM H. T. HOLDEN, OF BROOKLYN, NEW YORK, ASSIGNOR TO AMERICAN TELE- PHONE A ND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

CARRIER AMPLITUDE-CONTROL SYSTEM.

Application filed Mai-ch27, 1926. Serial m. 98,021.

This invention relates'to' signaling systerns, and more particularly to arrangements in such systems for controlling the ampli- 'tu'de of the carrier wave in accordance with the amplitude of low frequency signals.

In radio and carrier telephone and. telegraph systems in which the carrier frequency is transmitted modulated by low frequency signals, especially in systems in which the amplitude of the carrier frequency received the receiving station increases as the ampli-' tude of the carrier frequency increases.

When the carrier frequency becomes modulatcd by'low frequency si nals, the noise 5 may he comparatively sma il; yet it is desirable to inimize the noise during the silent interva s in the program, or durin the intervals when the amplitude of the ow fre: quency signals is smaller thanv usual. In this invention there is disclosed asys; tem in which the amount of carrier radiated depends upon the level of the. low 'frequenc signalssnpplied to the system to a consi erubly greater extent than is ordinarily the It is an object of this invention to vary the amplitude of the carrier wave inversely with variations in the amplitude of low frequency signals. In other words, it is an ob'ect of the invention to decrease the amplituce of the carrier wave as the level of the low frequency signals decreases, and vice versa.

It is an object of this invention-to reduce .the amount of noise which would normally be brought in at a radio receiver when a large amount of carrier is being transmitted during the quiet intervals in the program.

It is a further object of this invention to provide a suflicient amount of carrier to the system to satisfactorily demodulate one or both of the side bands that are transmitted in a signalin system without objectionable distortion wiien' relatively high levels of news, music, or the like are being trans mitted.

And it is a further object of this inven* tion to raise the energy level of the side band at the same time that the energy level of the carrier is lowered, and vice versa, in a system in which a single side band is transmitted so as to secure the samevariation in level upon demodulation at the. receiving end that exists at the transmitting end upon modulation. I

.Qther objects and features of this invention will be better understood .from the detailed description hereinaf ter following,-

when read in connection with the accompanym idrawing showing only one embodiment 0? the invention suitable for cont-rolling the carrier frequency in accordance with low frequency signals.

Referring to the drawing, the reference character L represents leads connected to.

a low frequency signalin source, which may be, for example, a'voice equency s. naling source. A trunsformerT connects t is low fre uency signaling source to a balanced mo ulator haying two threeelement ther-.

anionic tubes M and M each having a filament, a grid and a plate, the filaments being heated by, a common battery B The tubes M, and M are preferably similar and equal in their structure and characteristic and may be replaced by a singleduplex tube having a single filament, two grids and two plates. The input circuits of these tubes M and M, are arranged in parallel with respect to the secondary winding of a transformer Ti, which is in series with the battery B in the common conductor of the inputcircuits of these tubes The secondary of the transformer T consists of two windings which may, for example, be wound in the same direction, one in the input circuit of the tube M and the other in the input circuit of the tube M The output circuits of these tubes are also connected through the primary winding ot a transformer T in parallel with respect to a common plate battery B A filter F is connected to the secondary winding of the transformer T,,.' This filter may be-of a1? well-known-type which will the United States patent to G. A. Campbell, No. 1,227,113, dated May 22, 1917.

The leads L, are also connected through transformer T, to an amplifier A, for am- )lifying the si nals coming in over the leads l This amplifier may be of any type well known in the art, preferably a vacuum tube amplifier. An oscillator is represented by the reference character 0, which oscillator supplies the carrier fre uency to be employed in the system. Tiis oscillator may be of any well-knownv type, preferably a vacuum tube oscillator, setting up oscillations of constant frequency.

A detector 1) and an am ililier A each having a plate, a filament an a grid, are together employed for controlling the ampli tude of the carrier frequency in accordance with the amplitude of low frequency signals, as will be explained more fully in the description hereinafter following.

The output of the amplifier A, is connected to the grid and filament of the detector D through a condenser C, and a resistance R,. The output of the oscillator O is connected to the input circuit of the amplie ficr A by the flow of. current in the circuit including the primary windiu of the transformer T,, which is in parallc with a resistance R", a resistance R, a condenser G a resistance R a batter B and ground, the resistance R, and the attery B being directly connected between the grid and filament of the amplifier A The anodes of the amplifier A,, the detector D and the amplifier A, are supplied with direct current potential from batteries 13,, B, and B, through the primer windingof a transformerq a choke coil and theprimary winding o a transformer T respectively. The plate of the detector D is directly connected to the grid of the amplifier A, through a condenser C, which prevents the otential of the battery 13 from being applie to the grid of the amplifier A The output of the am plifier A. is connected tlirou h transformer filter F, which may e of any .wellknown type, preferably ofthe ty disclosed in the patent to G. A. Campbel above referred to, freely transmitting the carrier frequency of the oscillator O and substantially suppressing all other frequencies. The output of the filter F is connected with the out ut circuit L, in series with the output of t e filter F The condenser C, and the resistance R, are of such values thatfthe potential ap lied to the grid, and filament of the detector varies with/the syllabie frequency (the amplitude. of the low frequency signals). Consequent-' ly a corresponding variation will take place in the plate-filament impedance of the detector D. But the input impedance of the amplifier A, depends upon the impedance of resistance R,, and reslstance R, is in parallei with the plate-filament impedance of the detector D. Inasmuch as the platefilament impedance of the detector D- varies with the syllabic frequency (the amplitude of the low frequency signals), averaged over a period determined by the time constant of the condenser C and the resistance R hence the greater the amplitude of the low frequency signals becomes, the greater will be the platefilament impedance of the detector D, and the greater will be the input impedance of the amplifier A Consequently the amplitude of the carrier frequency will increase as the amplitude of the low frequency si nals increases, and vicc'versa. Thus it wi be noted that'the detector D and the amplifier A together form an arrangement which is employed for controlling the am litude of the carrier frequenc transmitted liy the amplifier A, in accor unce with the low frequency signals coming in over the leads L.

The carrier frequency is also transmitted to the primary winding of the transformer T by virtue of the presence of the primary winding of the transformer T in the circuit including the output of the carrier fr uency oscillator O, the resistance R the con enser C the resistance R and ground. Some of the current in the input of the oscillator O is shunted by a resistance R, which is connected across the terminals of the oscillator 0 through ground. By virtue of the connection between the output of the oscillator O and the transformer T the grids of the tubes M, and M are both in phase with respect to the carrier frequency transmitted by the oscillator Q. The oscillations of the carrier frequency therefore flow in opposite directions in the input circuits of the tubes M and M As the low frequency signals come in over the leads L,, the grid of one of the tubes M, or M, becomes positive while the grid of the other becomes negative, so that the resistance of one increases while the resistance of the other decreases. This then causes an increase in the amplitude of the oscillations of the carrier frequency flowing in the input circuit of the tube in-which the grid is positive and a corresponding decrease in the amplitude of the oscillations of the carrier frequency in the input circuit of the tube in, which the grid is negative, and the tubes M, and M, are actuated only during the time that low frequency signals are coming in over the leads L,.

-As has already been stated, the impedance of the resistanbe R, increases when the amplitude of the low frequency signals corning in over the leads 11,:increases, and vice versa. Therefore, when the impedance of the resistance R, increases, less current will flow in the circuit interconnecting the carrier frequency oscillator. the primary winding of the transformer T the resistance 3-,, the condenser C the resistance R, and

ground. Therefore, the products of modulation resulting from impressing the carrier frequency of the oscillator O and the low frequency signals coming in from the leads L upon the balanced modulator decrease. Obviously, when the impedance of the resistance R decreases, the products of modulation will increase. Since in this particular arrangement the filter F is so related to the balanced modulator as to transmit one side band resulting from modulation, the energy level of that side band will increase and decrease asthe amplitude of the carrier frequency passing the amplifier A decreases and increases, respectively. The output of the filter F Which is the aboveanentioned side band, and the output of the filter F which is the carrier frequency of the oscillator O, are united in the receiving circuit L,, the energy level of the side band trans mitted by the filter F varying inversely with the amplitude of the carrier frequency transmitted by the filter F \Vhile the arrangement as disclosed herein involves the transmission of a single side band along with the carrier frequency, the principles of thisinvention may be applied equally well to a system in which both side bands are transmitted along with the carrier frequency.

While the arrangements of this invention have been shown with particular reference to the control of a carrier frequency employed in a transmission system in accordance. with low frequency signals, it is to be understood that the invention may be embodied in other systems, and in other and different organizations without departing from the spirit of the invention and the scope of the appended claims.

\Vhat is claimed is:

1. In a signaling system, a source of signaling oscillations, a high frequency wave, means for controlling the amplitude of said high frequency wave in accordance with the amplitude of the oscillations of said signaling source, and a modulator for superimposing said signaling oscillations on said controlled high frequency wave, the amplitude of said high frequency wave impressed on said modulator decreasing as the amplitude of the oscillations of said signaling source increases, and vice versa.

2. In a signaling system, a source of voice frequency oscillations, a high frequency wax'e, a pair of thermionic devices, means for impressing said high frequency wave and said voice frequency oscillations on said pair of thermionic devices so that the amplitude of said high frequency wave is controllel by the amplitude of said voice frequency oscillations. means for "arying the amplitude of the high frequency Wave inversely with variations in the amplitude of said voice frequency oscillations, a modulafor for modulating said high frequency wave controlled in the latter manner by said voice frequency oscillations, a band filter for suppressing said high frequency wave and one side band thereof resulting from modulation and transmitting the.other side band thereof, and means for combining the high frequency wave controlled by the said pair of thermionic devices with the side band of the high frequency wave transmitted by said filter.

3. In a signaling system, a source of voice frequency oscillations, at high frequency wave, means for impressing said high frequency wave and said voice frequency oscillations on a pair of thermionic devices so as to control the amplitude of said high frequency wave by the amplitude of said voice frequency oscillations, and a modulator for modulating said high frequency wave by said voice frequency oscillations, the amplitude of said high frequency wave modulated by said modulator varying inversely with the amplitude of said voice frequency oscillations.

4. In a signaling system, a source of voice frequencies, a carrier frequency, means for varying the amplitudeof the carrier frequency in accordance with the voice frequencies, means for varying the amplitude of the carrier frequency inversely with variations of the voice frequencies, a modulator for modulating said carrier frequency varied in the latter manner b said voice frequencies, a band filter for su stantially supressing said carrier frequency and one side band and for transmitting the other side band resulting from modulation, and means for combining and transmitting said carrier frequency controlled in accordance with the voice frequencies and the side band thereof resulting from modulation.

5. In a system for the transmission of signals, a source of voice frequencies, a carrier frequency, means for decreasing the amplitude of the carrier frequency as the amplitude of the voice frequencies increases, and vice versa, means for modulatin the said carrier frequency varied in the a ove manner by substantially suppressing the carrier frequency and one side band and for transmitting the other side band resulting from modulation, means for increasing the amplitude of the carrier frequency as the amplitude of the voice frequency increases, and vice .Versa, and means for combining the carrier frequency controlled in the latter manner with the side band of the carrier frequency resulting from modulation.

6. In a signaling system, a source of signaling oscillations of varying amplitude, a source of high frequency oscillations, a thermionic device upon which said high frequency oscillates are impressed, said thersaid voice frequencies, means for.

mionic device being employed for regulating the amplitude of said high frequency oscillations impressed thereon in accordance with the signaling oscillations, means for modulating the regulated high frequency o.:cillations with the signaling oscillations, means for deriving one side band of said modulated high frequency oscillations from the products of modulation, and means for recombining the regulated high frequency oscillations and the side band thereof resulting from modulation.

7. In combination, a source of low frequency signals of varying amplitude, a high frequency oscillator, two three-element vacuum tubes each having a plate, a filament and a grid, :1 source of low frequency signals being connected to the grid and filament of one of the vacuum tubes so as to effect changes in the potential of the grid with respect to the filament of that tube at a rate corresponding to variations in the amplitude of the low frequency signals, the high frequency oscillator being connected to the grid and filament of the other three-electrode vacuum tube, said tubes being so connected that changes in the plate-filament impedance of the tube to which the source of low frequency signals is connected effect corresponding changes in the potential of the grid with respect to the filament of the tube to which the high frequency oscillator is connected, and a modulator for combining the low frequency signals with the wave of the carrier frequency oscillator so as to produce a side band of said wave, said side band varying inversely with the amplitude variations of the carrier frequency brought about by said vacuum tubes.

8. In a system for controlling the amplitude of a carrier frequency in accordance with low frequency signals of varying am plitude, in combination, two three-electrode vacuum tubes each having an anode, a cathode and a grid, the source of low frequency signals being connected to the grid and cathode of one of the vacuum tubes, the carrier frequency generator being connccted to the grid and cathode of the other vacuum tube, said vacuum tubes being so related that variations in the potential be tweenthe grid and cathode of the vacuum tube to which the source of low frequency signals is connected cause corresponding variations in the amplitude of the current flowing between the anode and cathode of the vacuum tube to which the carrier frequcncy generator is connected and a modulator for modulating the low frequency signals with the carrier frequency the amplitude of the carrier frequency at said modulator varying inversely with the changes in amplitude of the low frequency signals.

9. In a system for controlling the ampli tude of a carrier frequency wave, in com bination, a source of low frequency signals of varying amplitude, a carrier frequency generator, two three-electrode vacuum tubes each having an input circuit and an output. circuit, the source of low frequency signals being connected to the input circuit of one of the vacuum tubes and a carrier frequency generator being connected to the input circuit of the other of the vacuum tubes, said vacuum tubes being so related that variations in the potcntialin the input circuit of the vacuum tube to which the source of low frequency signals is connected cause corresponding variations in the current in the output circuit of the vacuum tube to which the carrier frequency generator is connected, and a modulator to which the source of low frequency signals and the carrier frequency generator are connected, the amplitude of the carrier frequency at said modulator varying inversely with the amplitude variations of. the low frequency signals, said modulator modulating the low frequency signals and the carrier frequency.

10. A system for controlling the amplitude of a carrier frequency in accordance with low frequency signals of varying amplitude, comprising two three-electrode vacuum tubes placed in tandem with respect to the source of low frequency signals so that the plate and filament of the first vacuum tube are in parallel relationship with the grid and filament of the second vacuum tube, the source of low frequency signals being connected to the grid and filament of the first vacuum tube and the carrier frequency oscillator being connected to the grid and filament of the second vacuum tube, means connected to the second vacuum tube for freely transmitting the carrier frequency controlled as to its amplitude and substantially suppressing all other frequencies, and a modulator for modulating the low frequency signals with the controlled carrier frequency.

In testimony whereof, I have signed my name tothis specification this 26th day of March, 1926.

WILLIAM H. T. HOLDEN. 

